This invention relates to optical imaging systems employing a series of reflecting elements and, more particularly, to a system having multiple reflections from any one of a set of mirrors for exclusion of light from sources outside the viewing angle of the system, and for rendering the system more compactly.
Optical imaging systems are employed over various portions of the spectrum including both the visual and infra-red portions of the spectrum. Of particular interest is the imaging of objects at long distances by telescope. Optical elements including both lenses and mirrors may be employed in telescopes, the mirrors being advantageous in regions of the spectrum in which totally transparent lens material may not be available, or where material transparent for all wavelengths of interest is not available. The mirrors are also preferred for their optional choice of thickness which can result in greater rigidity and the maintenance of dimensional stability in the case of very large elements, and allow for situations wherein a large amount of curvature is required in an optical surface.
A problem attendant through the use of telescopes is manifested in the viewing of an object which is a relatively weak source of light. This is especially so in the situations wherein the weak object is located nearby, in terms of angular spacing, to a relatively strong source of light. Light from the strong source may enter the optical system and mask the image of the weak source. A narrow viewing angle lessens the amount of light received from the strong source.
A narrow viewing angle can be obtained in a telescope constructed with mirrors by reflecting the rays of light back and forth between a pair of the mirrors, such as between a primary mirror and a secondary mirror which are curved to have focussing power. The multiple reflections permit reduction in the physical size of the telescope to provide the advantage of a more compact telescope. One such multiple-reflection optical system is shown in FIG. 1 of U.S. Pat. No. 3,527,526 issued in the name of Silvertooth. This patent is incorporated by reference in its entirity herein. However, these optical systems suffer a disadvantage in that still further isolation from the rays of the strong source is often desirable.